1. Field of the Invention
This invention relates generally to the field of screen printing squeegees. More particularly, the present invention relates to a screen printing squeegee system using a quick-release clamping means to hold a squeegee having at least a diamond shaped cross section portion. This combination provides for quick change or rotation of the squeegee and utilization of a plurality of sides or corners of the squeegee.
2. Background of the Invention
In screen printing, there are many variables which effect the printing quality. For example the squeegee's durometer, sharpness and angle must be optimized to obtain good results. Most common squeegee holders are similar to the squeegee holder 15 shown in FIG. 1. In the squeegee holder of FIG. 1, a squeegee 20 having a wide rectangular cross section is clamped between two clamping plates 24 and 26 to expose a printing bottom edge of the squeegee 20. The squeegee 20 is held in place by aligning the squeegee 20 and then tightening a plurality of bolts 34 to secure the squeegee 20 in place. One of the most important factors determining print quality is the alignment of the squeegee with the substrate being printed.
For squeegee holders such as that shown in FIG. 1, tools are required to tighten bolts 34 whenever the squeegee is replaced. Whenever it is replaced, the alignment of the squeegee must be adjusted. To achieve quality printing with consistent results, a properly aligned edge 30 of the squeegee 20 is essential. If this edge is not even, uneven deposits of ink results with color variation and smearing or partial prints. Currently, this bottom edge 30 of the squeegee is aligned by eye. Since tools are required to adjust the alignment of the squeegee holder, adjusting the squeegee for proper alignment is cumbersome and time consuming.
Squeegees such as squeegee 20 are frequently sharpened to increase their life and avoid the need to adjust their alignment. However, the material that such squeegees are made of are frequently sensitive to the heat which might be generated during the sharpening process. After sharpening, the squeegee may no longer be protected against solvents such as acetone, alcohol and sometimes the ink itself. The result is often dimensional changes or premature softening of the squeegee material. Thus sharpening is not really advisable.
Referring to FIG. 2, the squeegee holder and squeegee of FIG. 1 is shown in operation. The entire assembly 15 is tilted to the right at approximately 15.degree. (generally between 10.degree. and 20.degree. ) as indicated by angle 40. Downward pressure is then applied to the assembly 15 as the squeegee is pulled across a screen 44 in the direction of travel shown. The squeegee 20 wipes ink across the screen 44 which is anchored at two points 46 and 48. Ink is then deposited through mesh openings in the screen 44 on to the substrate 50 being printed. In the case of the present invention, the substrate 50 is a compact disc. This printing process is used to produce label information and decorative patterns or images on the compact disc.
Referring to FIG. 3, the angle 40 is typically set at between 10.degree. and 20.degree., with 15.degree. being a typical angle. The downward pressure exerted on the squeegee holder deforms the squeegee edge 30 to produce a true printing angle 60 of approximately 40.degree. at the contact edge 30 due to the deformation of the squeegee.
Screen 44 is a fabric screen with openings in the mesh through which ink is deposited on the substrate 50. The quantity of ink deposited is controlled by the mesh opening, thickness of the fabric, thickness of the emulsion, snap-off distance between the screen and substrate, squeegee wiper durometer, squeegee wiper blade sharpness and squeegee angle. The downward pressure is generally created manually or with air pressure or other mechanical pressure.
Because the top edge 32 of the squeegee 20 is subjected to both the downward pressure exerted on the squeegee holder and the pressure applied by clamping plates 24 and 26, it is possible that the top edge 32 will be damaged while the bottom edge 30 is in use. This could require the top edge 32 be sharpened before use or simply result in the top edge 32 not being usable. In the latter case, the useful life of the squeegee is cut in half. In the former case, the deleterious effects of sharpening described above as well as the additional time required for accomplishing the same must be endured.
Furthermore, with the system shown in FIGS. 1 through 3, the critical adjustment of the squeegee 20 is carried out manually by eye and additional tools are required. This is a time consuming process which can obviously result in inconsistent quality.
U.S. Pat. No. 4,122,771 discloses a squeegee holder wherein the squeegee is held by two jaws at a canted diagonal alignment, so that a corner edge of the squeegee is exposed beneath the jaws. While this type of holder remedies some of the alignment problems of previous devices, this holder suffers from the disadvantage that the two jaws are held together by screws or bolts, such as bolts 34 in FIG. 1. Again, this necessitates the use of additional tools and time when the squeegee is to be changed. Furthermore, such a holder suffers from the additional disadvantage that the corners of the three sides not exposed beneath the jaws (i.e., the corners not being used for printing) are all in contact with the jaws. Thus, these sensitive corner areas are subjected to the applied downward force and/or the forces holding the jaws together and that applied during use. As a result, these corners can be damaged before being used, sharply decreasing the possible usable lifespan of the squeegee, or requiring that the squeegee be subjected to the deleterious effects of sharpening before using a damaged corner.
The present invention seeks to improve the print quality in an improved squeegee holding system which provides consistent alignment and rapid change of the squeegee as well as for extending the possible useful life of the squeegee.